Lithography is a printing method which effectively utilizes the property of water and oil in that they essentially do not mix. A printing plate for lithography comprises a water-receptive and oil-base-ink-repellent portion and a water-repellent and oil-base-ink-receptive portion, with the former constituting a nonimage area and the latter constituting an image area.
In general, assistants for a lithographic printing plate, such as printing plate protective agents, printing plate cleaners, image erasers, sensitization protective agents and gum removers, are used in order to protect the nonimage areas. Water-soluble polymers having a film formability have hitherto been used as an indispensable ingredient of the assistant.
For example, in the final step of lithography, a plane protective agent is coated (this procedure being usually referred to as "gumming") for the purpose of protecting the nonimage areas.
The plate protective agent is used in order to maintain the hydrophilicity of the nonimage areas by taking advantage of the film formability and for other purposes such as correction in image areas, such as retouching or elimination, storage in a period between after plate making and before initiation of printing or storage until reuse, prevention of contamination caused by deposition of fingerprints, fats and oils, dust, etc., during handling at the time of mounting the plate in a printing machine, protection against occurrence of flaws and prevention of contamination by oxidation.
In most cases, the conventional plate protective agent comprises a solution of a water-soluble polymer, such as gum arabic, cellulose gum or a polymer having a carboxyl group in its molecule and optional additives, such as pH adjustors and preservatives.
In the plate protective agent, when gum arabic, which is a naturally occurring plant gum substance, is used as the water-soluble polymer, it exhibits an excellent film formability.
On the other hand, as well known in the art, in the production of a photosensitive lithographic printing plate, removal of an unnecessary image, that is, the so-called "image erasing," is carried out subsequent to the step of forming an image. In particular, in a positive-working lithographic printing plate, unnecessary images are likely to occur due to traces of an image of an original film and foreign matter such as contaminants, which renders the image erasing indispensable.
In the image erasing of the lithographic printing plate, in addition to removal of unnecessary images, the surface of the substrate at its portions where the images have been removed should be rendered hydrophilic so that these portions do not receive the ink during printing. However, useful image erasing techniques, which can satisfy the above requirements, are very few.
Examples of a useful image erasing technique include a technique where the unnecessary images are physically rubbed out with a stone rod, a technique where the images and the substrate are subjected to etching with a strong alkaline solution to remove the unnecessary images and a technique where a solution containing an organic solvent capable of dissolving an image and an acidic substance, such as hydrofluoric acid, is used to dissolve unnecessary images and, at the same time, to etch the substrate to impart hydrophilicity to the substrate in its nonimage areas. Although these techniques are useful, the former method has drawbacks including the fact that a large area cannot be treated and contamination occurs during printing. On the other hand, the two latter methods have several drawbacks including the fact that necessary images in their fine portions are damaged, harmfulness to human body and skin is high and there occurs a problem of waste water treatment. For this reason, the above techniques are not always satisfactory.
in recent years, use of an image eraser comprising a water-soluble polymer, an organic solvent, a surfactant and a mineral acid has been exclusively adopted as a method useful for reducing the above drawbacks. In the method wherein such an image eraser is used, when gum arabic, which is a naturally occurring plant gum substance, is used as the water-soluble polymer, it exhibits an excellent film formability.
Further, as described above, also when gum arabic is used as the water-soluble polymer used in a plate cleaner for protecting nonimage areas, a sensitization protective agent or a gum remover, it exhibits an excellent film formability.
Further, in lithography, dampening water for lithography is used. The dampening water for lithography wets nonimage areas to increase the interfacial chemical difference between the image area and the nonimage area, thereby enhancing the ink repellency of the nonimage area and the ink receptivity of the image area.
Also in the dampening water for lithography, a water-soluble polymer having a film formability is used as an indispensable ingredient. In this case, when gum arabic, which is a naturally occurring plant gum substance, is used as the water-soluble polymer, it exhibits an excellent film formability as dampening water for lithography.
As described above, gum arabic, which has hitherto been used as a water-soluble polymer in assistants for lithography, exhibits an excellent suitability for all the assistants. However, the supply thereof is susceptible to weather in production countries, so that the price fluctuation is large. For this reason, in recent years, naturally occurring gum substances, which can be stably supplied, have become strongly desired in the art.
It is noted that chemically modified starch or the like has been developed as an alternative to the gum arabic. It, however, has a poor capability of desensitizing the nonimage area, so that it is not always satisfactory.